Micromechanical device with elastic assembly having variable elastic constant

ABSTRACT

A micromechanical device includes a semiconductor body, a first mobile structure, an elastic assembly, coupled to the first mobile structure and to the semiconductor body and adapted to undergo deformation in a direction, and at least one abutment element. The elastic assembly is configured to enable an oscillation of the first mobile structure as a function of a force applied thereto. The first mobile structure, the abutment element and the elastic assembly are arranged with respect to one another in such a way that: when the force is lower than a force threshold, the elastic assembly operates with a first elastic constant; and when the force is greater than the threshold force, then the first mobile structure is in contact with the abutment element, and a deformation of the elastic assembly is generated, which operates with a second elastic constant different from the first elastic constant.

BACKGROUND Technical Field

The present disclosure relates to a micromechanical device with an elastic assembly having a variable elastic constant.

Description of the Related Art

As is known, there is a desire to detect and measure effectively accelerations and shocks via sensors having small dimensions that can be easily integrated. Common applications include monitoring of shocks in electronic devices, such as mobile phones and smartwatches, for example for detecting car accidents or cases of people falling on the ground as a result of feeling unwell or fainting or because they suffer from an illness.

Currently, available on the market are low-G sensors (such as accelerometers and gyroscopes) adapted to detect low accelerations (for example, with a full-scale range of 16 g or 32 g) and high-G sensors adapted to detect high accelerations (for example, with a full-scale range of 128 g). The former are used for detecting usual movements of operators provided with the electronic device that integrates the sensors (such as approach of the mobile phone to the operator's ear, or movement of the wrist to which the smartwatch is connected), whereas the latter enable detection of high-intensity accelerations (and therefore, anomalous events).

In order to enable an electronic device to detect both low accelerations and high accelerations, known solutions envisage integration in the electronic device of both types of accelerometer. However, the simultaneous presence in a same electronic device of two different accelerometers entails disadvantages such as a larger number of pads necessary and a higher complexity in the control circuitry (for example, dedicated ASICs, PCBs, or CPUs, etc.), and more in general a greater integration area, a lower portability of the electronic device, and a higher manufacturing cost.

Given the drawbacks, the patent document US2006/107743A1 discloses the structure of an accelerometer that enables implementation of two different sensitivities in respective and different operating modes. In particular, the above accelerometer includes, in one embodiment (designated in FIG. 1A by the reference number 1 a), a seismic mass 2 fixed to a first end 3 a of a first spring element 3 having an elongated shape. The first spring element 3 is moreover fixed to a support 5 at a second end 3 b thereof, opposite to the first end 3 a. A second spring element 4, having an elongated shape and having a first end 4 a and a second end 4 b opposite to one another, is moreover fixed to the support 5 at its second end 4 b. The first and second spring elements 3, 4 have a main extension along a first direction orthogonal to a main extension of the support 5 (for example, orthogonal to a surface of the support 5), and are therefore set parallel to one another and with respect to the first direction. In addition, they are aligned with one another in a direction perpendicular to a second direction orthogonal to the first direction. In use, the first spring element 3 is deflected in a direction perpendicular to its main extension by a force F (for example, a force of gravity) acting in the second direction. When the force F is equal to a threshold force F_(th), the first spring element 3 presents a deflection such that it comes into contact, at a portion of a bottom surface 3 c thereof, with the first end 4 a of the second spring element 4. For forces F lower than the threshold force F_(th), the accelerometer 1 a has a first value K₁ of elastic constant (that depends just upon the characteristics of the first spring element 3); for forces F higher than the threshold force F_(th), the accelerometer 1 a has, instead, a second value K₂ of elastic constant (that depends upon the characteristics of the second spring element 4) greater than the first value K₁. The presence of the second spring element 4 therefore enables modification of the stiffness of the accelerometer 1 a as a function of the force F applied.

According to a different embodiment of the accelerometer, disclosed in the same patent document US2006/107743A1 (designated in FIG. 1B by the reference number 1 b), the seismic mass 2 is connected to the support 5 via a third spring element 7, which that has a pyramidal tapering from the end in contact with the support 5 to the end in contact with the seismic mass 2. The shape of the third spring element 7 makes it possible to obtain, in use, a non-linear profile of the elastic constant, and therefore a stiffness of the accelerometer 1 b that varies as a function (in particular, logarithmically) of the force F applied.

However, the accelerometer 1 a presents a low mechanical stability since, during an event of shock or in any case of marked acceleration, the spring elements 3, 4 may be overstressed and undergo damage or failure due to mutual contact. Instead, in the case of the accelerometer 1 b, the real plot of stiffness is difficult to predict theoretically in an accurate way since it depends upon a multiplicity of structural factors, factors of use, and process factors.

BRIEF SUMMARY

In various embodiments, the present disclosure provides a micromechanical device that will overcome the problems of the prior art.

In one or more embodiments of the present disclosure, a micromechanical device is provided that includes a semiconductor body; a first mobile structure, having a first mass, configured to oscillate relative to the semiconductor body in a direction belonging to a plane; an elastic assembly, having an elastic constant, mechanically coupled to the first mobile structure and to the semiconductor body, and configured to expand and contract in the direction; and at least one abutment element. The elastic assembly is configured to enable the oscillation of the first mobile structure as a function of a force applied to the first mobile structure in the direction. The first mobile structure, the abutment element, and the elastic assembly are arranged with respect to one another in such a way that: when the force applied to the first mobile structure is lower than an abutment-force threshold, then the first mobile structure is not in contact with the abutment element, and the elastic assembly operates with a first elastic constant; and when the force applied to the first mobile structure is greater than the abutment-force threshold, then the first mobile structure is in contact with the abutment element and, under the action of the applied force, a deformation of the elastic assembly is generated such that the elastic assembly operates with a second elastic constant different from the first elastic constant.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, a preferred embodiment thereof is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:

FIGS. 1A and 1B are cross-sectional views of respective accelerometers of a known type;

FIG. 2 is a top view of a micromechanical device, according to one embodiment of the present disclosure;

FIGS. 2A and 2B illustrate the micromechanical device of FIG. 2 in respective operating modes;

FIG. 3 is a top view of a further embodiment of the micromechanical device according to the present disclosure;

FIGS. 3A and 3B are top views of the micromechanical device of FIG. 3 , in respective operating modes;

FIG. 3C is a top view of a further embodiment of the micromechanical device according to the present disclosure;

FIG. 4 is a top view of a further embodiment of the micromechanical device according to the present disclosure;

FIGS. 4A and 4B are top views of the micromechanical device of FIG. 4 , in respective operating modes;

FIG. 5 is a top view of a further embodiment of the micromechanical device according to the present disclosure;

FIGS. 5A and 5B are top views of the micromechanical device of FIG. 5 , in respective operating modes;

FIG. 6A is a graph that represents an electrical signal generated at output by the micromechanical device of FIG. 3 as a function of an acceleration to which the micromechanical device is subjected in use; and

FIG. 6B is a graph that represents the plot of the stiffness of the micromechanical device of FIG. 4 as a function of a displacement of a sensing mass belonging to the micromechanical device with respect to a resting position.

DETAILED DESCRIPTION

In particular, the figures are illustrated with reference to a triaxial cartesian system defined by a first axis X, a second axis Y, and a third axis Z, orthogonal to one another.

In the ensuing description, elements that are common to the different embodiments are designated by the same reference numbers.

Moreover, in the ensuing description, the term “substantially” is used to refer to a property that is considered verified to a first order. For instance, if two elements moving with respect to a reference point are said to be “substantially” fixed with respect to one another, it is meant that, even though there may exist a relative movement between them, this relative movement is negligible as compared to the movement with respect to the reference point (for example, the relative movement is less than 5% of the movement of each element with respect to the reference point). Likewise, if an element is said to present a “substantially” zero deformation along one axis, it is meant that a possible deformation of the element is negligible as compared to the extension of the element itself along the aforesaid axis (for example, the deformation is less than 5% of the extension of the element along said axis).

FIG. 2 shows a micromechanical device 50 configured to detect accelerations (hereinafter also referred to as sensor 50), according to one embodiment. FIG. 2 is a top view (i.e., in the plane XY) of the sensor 50. Illustrated in FIG. 2 are just the elements useful for an understanding of the present embodiment, and elements or components that, albeit present in the finished sensor, are not important for the present disclosure are not illustrated.

The sensor 50 comprises a semiconductor body 51 of semiconductor material, such as silicon (Si), having a surface 51 a extending parallel to a first plane XY defined by the first axis X and by the second axis Y (i.e., the third axis Z is orthogonal to the surface 51 a). The sensor 50 further comprises a first mobile structure 53 having a first mass M₁, and a second mobile structure 55 having a second mass M₂, greater than the first mass M₁. In what follows, the first mobile structure 53 will be referred to as “first seismic mass”, and the second mobile structure 55 will be referred to as “second seismic mass”.

Both the first seismic mass 53 and the second seismic mass 55 are, for example, of semiconductor material (such as silicon or polysilicon) and extend parallel to the surface 51 a of the semiconductor body 51, at a different height, along the axis Z, with respect to the height of the surface 51 a.

The first seismic mass 53 is physically coupled to the semiconductor body 51 via a first spring assembly 57 (in detail, a first spring, or first elastic element, 57 a of the first spring assembly 57 and a second spring, or second elastic element, 57 b of the first spring assembly 57), whereas the second seismic mass 55 is physically coupled to the semiconductor body 51 via a second spring assembly 59 (in detail, a first spring, or first elastic element, 59 a of the second spring assembly 59 and a second spring, or second elastic element, 59 b of the second spring assembly 59). Both the first spring assembly 57 and the second spring assembly 59 are, for example, of semiconductor material (such as silicon or polysilicon) and undergo deformation (i.e., they lengthen/shorten) along the first axis X. In other words, both the first spring assembly 57 and the second spring assembly 59 have respective axes along which deformation occurs parallel to the first axis X. In addition, deformation of the first and second spring assemblies 57, 59 occurs along a same direction of deformation 60.

In the embodiment described by way of example, both the first portions 57 a, 59 a and the second portions 57 b, 59 b of the elastic elements 57, 59 are serpentine springs. In particular, such serpentine springs are of a planar type and are obtained with MEMS technology (i.e., by methods of machining of semiconductors). In greater detail, said serpentine springs may include first portions, which extend parallel to one another and to the second axis Y, and second portions, which extend parallel to one another and to the first axis X. The first and second portions are connected to one another and are mutually arranged so as to form a serpentine path: each first portion is connected, at its ends that are opposite to one another along the second axis Y, to respective second portions; and each second portion is connected, at its ends that are opposite to one another along the first axis X, to respective first portions, except for two second portions (each of which is set at a respective end of said path along the first axis X and is joined to just one respective first portion).

Each spring 57 a, 57 b of the first spring assembly 57 has a respective first end 57 a′, 57 b′ and a respective second end 57 a″, 57 b″, which are opposite to one another along the first axis X. Each spring 59 a, 59 b of the second spring assembly 59 has a respective end 59 a′, 59 b′ and a respective end 59 a″, 59 b″, which are opposite to one another along the first axis X.

In particular, the distance, measured along the axis X, between the end 57 a′ and the end 57 a″ of the first spring 57 a of the first spring assembly 57, is identified by the reference L_(1a). The distance, measured along the axis X between the end 57 b′ and the end 57 b″ of the second spring 57 b of the first spring assembly 57, is identified by the reference L_(1b). The distance, measured along the axis X between the end 59 a′ and the end 59 a″ of the first spring 59 a of the second spring assembly 59, is identified by the reference L_(2a). The distance, measured along the axis X between the end 59 b′ and the end 59 b″ of the second spring 59 b of the second spring assembly 59, is identified by the reference L_(2b).

The springs 57 a, 57 b of the first spring assembly 57 have respective first elastic constants K₁ (having the same value), and the springs 59 a, 59 b of the second spring assembly 59 have respective second elastic constants K₂ (having the same value as one another, but different from K₁, for example greater than K₁).

In the embodiment of FIG. 2 , two springs 57 a, 57 b are present so that the equivalent elastic constant of the first spring assembly 57 is given by 2K₁, and two springs 59 a, 59 b are present so that the equivalent elastic constant of the second spring assembly 59 is given by 2K₂. In general, for any number N1 of springs of the first spring assembly 57, the equivalent elastic constant of the first spring assembly 57 is given by N1·K₁, and for any number N2 of springs of the second spring assembly 59, the equivalent elastic constant of the second spring assembly 59 is given by N2·K₂.

Each spring 57 a, 57 b of the first spring assembly 57 is coupled, via the respective end 57 a′, 57 b′, to a respective first fixing element 64′ coupled to the surface 51 a of the semiconductor body 51 (in particular, each first fixing element 64′ is fixed with respect to the surface of the semiconductor body 51). Each spring 57 a, 57 b of the first spring assembly 57 is moreover coupled, at the respective end 57 a″, 57 b″, to the first seismic mass 53. In detail, in the embodiment discussed by way of example, the first seismic mass 53 has a first lateral surface 53 a and a second lateral surface 53 b opposite to one another along the first axis X, and each end 57 a″, 57 b″ is fixed with respect to a respective one between the first and second lateral surfaces 53 a, 53 b. Consequently, the first seismic mass 53 is set, along the first axis X, between the first and second springs 57 a, 57 b of the first spring assembly 57.

The second seismic mass 55 moreover has, in the view of FIG. 2 , a cavity 62 that houses within it the first seismic mass 53, the first spring assembly 57, and the first fixing elements 64′.

Each spring 59 a, 59 b of the second spring assembly 59 is coupled, via the respective end 59 a′, 59 b′, to a respective second fixing element 64″, which is in turn coupled to the semiconductor body 51 (in particular, the second fixing element 64″ is fixed with respect to the surface 51 a of the semiconductor body 51). Each spring 59 a, 59 b of the second spring assembly 59 is moreover coupled, at the respective end 59 a″, 59 b″, to the second seismic mass 55. In detail, in the embodiment discussed, the second seismic mass 55 has a first lateral surface 55 a and a second lateral surface 55 b opposite to one another along the first axis X, and each end 59 a″, 59 b″ is fixed with respect to a respective one between the first and second lateral surface 55 a, 55 b. Consequently, the second seismic mass 55 is set, along the first axis X, between the first and second springs 59 a, 59 b of the second spring assembly 59.

The first seismic mass 53 further includes a plurality of stopper elements 66 a (for example, in FIG. 2 , four stopper elements 66 a), and the second seismic mass 55 includes a respective plurality of housing elements 66 b (for example, in FIG. 2 , four housing elements 66 b). The stopper elements 66 a and the housing elements 66 b form an abutment assembly 66.

The stopper elements 66 a are protrusions of the first seismic mass 53, whereas the housing elements 66 b are respective portions of the second seismic mass 55, which present a respective cavity and/or recess. In the embodiment illustrated by way of example in FIG. 2 , both the stopper elements (protrusions) 66 a and the housing elements 66 b (cavities) have a substantially rectangular shape with a main extension parallel to the second axis Y. In particular, the first and second seismic masses 53, 55 are arranged in such a way that each stopper element 66 a extends within the cavity of each respective housing element 66 b or, in other words, each stopper element 66 a is partially surrounded by a respective housing element 66 b, to form a respective abutment assembly 66. In the absence of external forces acting along the axis X, each stopper element 66 a is not in contact with the respective housing element 66 b. Each stopper element 66 a has a first side wall 67 a and a second side wall 67 b, opposite to one another along the first axis X, whereas each housing element 66 b has a first side wall 67 c and a second side wall 67 d, opposite to one another along the first axis X and facing the first side wall 67 a and the second side wall 67 a, 67 b, respectively, of the respective stopper element 66 a. For each contact structure 66, the side walls 67 a, 67 c are at a distance equal to a first length L₁ from one another, whereas the side walls 67 b, 67 d are at a distance equal to a second length L₂ from one another.

Furthermore, the first seismic mass 53 includes one or more first electrodes 68 a (mobile electrodes), such as protrusions (for example, having a substantially rectangular shape in the plane XY), which, in use, displace in a way fixed with respect to the first seismic mass 53. One or more second electrodes 68 b (fixed electrodes) are fixed with respect to the semiconductor body 51, in particular to the surface 51 a.

Each of the second electrodes is further divided into a first portion 68 b′ and a second portion 68 b″, which are separate from one another. The first electrode 68 a extends between the first portion 68 b′ and the second portion 68 b″. In greater detail, each of the first electrodes 68 a faces, and is set between, the first portion 68 b′ of a respective second electrode 68 b and the second portion 68 b″ of said respective second electrode 68 b.

The first and second electrodes 68 a, 68 b form a measurement structure 68 of the sensor 50 adapted, in use, to detect in a capacitive way displacements along the first axis X of the first and second seismic masses 53, 55; these displacements are indicative of external forces (e.g., accelerations) that act on the sensor 50.

In particular, surfaces of the first electrode 68 a and of the first portion 68 b′ of the second electrode 68 b that directly face one another form a first capacitor 68′. Likewise, surfaces of the first electrode 68 a and of the second portion 68 b″ of the second electrode 68 b that directly face one another form a second capacitor 68″. The distance (along the axis X) between the first electrode 68 a and the first portion 68 b′ is designated by the reference d_(c1), whereas the distance between the first electrode 68 a and the second portion 68 b″ is designated by the reference d_(c2).

Moreover, first and second blocking elements 70′, 70″ are fixed with respect to the semiconductor body 51 (in particular, to the surface 51 a of the semiconductor body 51). FIG. 2 illustrates, by way of example, two blocking elements 70′ that are located at a distance L_(1block) along the first axis X from the first lateral surface 55 a of the second seismic mass 55. FIG. 2 likewise illustrates two blocking elements 70″ that are set at a distance L_(2block) along the first axis X from the second lateral surface 55 b of the second seismic mass 55.

In detail, in order to prevent direct contact between the first electrode 68 a and the portions 68 b′, 68 b″, the distance d_(c1) is designed to have a value such that d_(c1)>L₁+L_(1block), and the distance d_(c2) is designed to have a value such that d_(c2)>L₂+L_(2block).

During use of the sensor 50, the first electrode 68 a is biased at a first voltage V₁, and the second electrode 68 b is biased at a second voltage V₂. In particular, the first voltage V₁ and the second voltage V₂ are the same as one another (V₁=V₂).

Since, in use, the first and second distances d_(c1), d_(c2) vary as a function of the external force applied to the sensor 50 (which causes, as has been said, a displacement of the first seismic mass 53), it is possible to correlate the variation of capacitance of the capacitors 68′, 68″ to this applied force. The measurements of capacitance can be performed via techniques in themselves known, for example, via transimpedance amplifiers.

With the sensor 50 in the resting condition, no external force is applied to the sensor 50, and therefore both the first and second seismic masses 53, 55 are in the resting position.

The first seismic mass 53 has a first centroid B₁ and the second seismic mass 55 has a second centroid B₂. In the resting condition:

-   -   the first and second centroids B₁, B₂ coincide with one another         in the plane XY (B₁=B₂=B_(stat));     -   the first and second path lengths L₁, L₂ are the same as one         another (L₁=L₂=L_(stop));     -   the first and second distances d_(c1), d_(c2) are the same as         one another (d_(c1)=d_(c2)=d_(rest));     -   the first length L_(1a) and the second length L_(1b) are the         same as one another (L_(1a)=L_(1b)=L_(1rest));     -   the first length L_(2a) and the second length L_(2b) are the         same as one another (L_(2a)=L_(2b)=L_(2rest)); and     -   the first distance L_(1block) and the second distance L_(2block)         are the same as one another         (L_(1block)=L_(2block)=L_(blockmax)).

FIG. 2A shows the sensor 50 in a first operating condition, where an external force (having a first value F₁ lower than a threshold value F_(th)) is applied to the sensor 50.

In the present description, the external force is considered, by way of example, as a force acting in the direction of the first axis X (in FIG. 2A, from left to right); however, what is described hereinafter applies in a way in itself evident also to the case where the external force acts in the opposite direction. On account of its inertia, the first seismic mass 53 undergoes an apparent force equal to the external force applied to the sensor 50, but in an opposite direction (since the reference system illustrated in FIG. 2A and fixed with respect to the semiconductor body 51 is not inertial). The apparent force causes a relative movement of the first seismic mass 53 with respect to the semiconductor body 51. In particular:

-   -   in the plane XY, the first centroid B₁ is displaced along the         first axis X with respect to the position B_(stat) of the         centroids at rest, whereas the second centroid B₂ substantially         coincides with the position B_(stat) of the centroids at rest;     -   the first length L₁ is less than L_(stop), and the second length         L₂ is greater than L_(stop);     -   the first distance d_(c1) is less than the distance d_(rest),         and the second distance d_(c2) is greater than the distance         d_(rest);     -   the first length L_(1a) is less than the length at rest         L_(1rest), and the second length L_(1b) is greater than the         length at rest L_(1rest);     -   the first length L_(2a) and the second length L_(2b) are         substantially the same as one another and substantially equal to         the length at rest L_(2rest); and     -   the first distance L_(1block) and the second distance L_(2block)         are the same as one another and equal to the distance         L_(blockmax).

Therefore, considering by way of example N1=1, N2=1, in the first operating condition of FIG. 2A, the sensor 50 has a resonance pulsation ω_(res) according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{1}}{M_{1}}}$

FIG. 2B shows the sensor 50 in a second operating condition, where the external force applied to the sensor 50 has a second force value F₂ greater than, or equal to, the threshold value F_(th). For the same reasons as those described previously, the first seismic mass 53 is displaced with respect to the resting position of FIG. 2 , and the stopper elements 66 a are in abutment against the respective housing elements 66 b. In particular, the first centroid B₁ is displaced, in the plane XY along the first axis X, with respect to the position B_(stat) (centroid at rest) and the first seismic mass 53 is in direct physical contact with the second seismic mass 55 at the first side walls 67 a, 67 c of the stopper elements 66 a and of the housing elements 66 b. In this operating condition, also the second seismic mass 55 is displaced with respect to the semiconductor body 51, and to its resting position illustrated in FIG. 2 , under the thrust of the first seismic mass 53. Therefore, also the second centroid B₂ is displaced, in the plane XY along the first axis X, with respect to the position at rest B_(stat). Consequently, in the second operating condition, the second seismic mass 55 moves in a way fixed with respect to the first seismic mass 53. In particular:

-   -   the first length L₁ is zero and the second length L₂ is twice         the length L_(stop);     -   the first distance d_(c1) is less than the resting distance         d_(rest) (moreover, it is less than the first distance d_(c1) of         FIG. 2A), and the second distance d_(c2) is greater than the         resting distance d_(rest) (moreover, it is greater than the         second distance d_(c2) of FIG. 2A);     -   the first length L_(1a) is less than the first length at rest         L_(1rest) (moreover, it is less than the first length L_(1a) of         FIG. 2A), and the second length L_(1b) is greater than the first         length at rest L_(1rest) (moreover, it is greater than the         second length L_(1b) of FIG. 2A);     -   the first length L_(2a) is less than the second length at rest         L_(2rest) (moreover, it is less than the first length L_(2a) of         FIG. 2A), and the second length L_(2b) is greater than the         second length at rest L_(2rest) (moreover, it is greater than         the second length L_(2b) of FIG. 2A); and     -   the first distance L_(1block) is less than the maximum distance         L_(blockmax), and the second distance L_(2block) is greater than         the maximum distance L_(blockmax).

Therefore, considering by way of example N1=1, N2=1, in the second operating condition of FIG. 2B the resonance pulsation ω_(res) of the sensor 50 is obtained according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{1} + K_{2}}{M_{1} + M_{2}}}$

In particular, if the external force applied to the sensor 50 has a value greater than, or equal to, a maximum force value F_(max) (greater than the threshold force value F_(th)), the second seismic mass 55 is in abutment against the first blocking element 70′ at a portion of the first lateral surface 55 a of the second seismic mass 55. In other words, the first distance L_(1block) is zero, and the second distance L_(2block) is twice the maximum distance L_(blockmax). The blocking elements 70 therefore enable limitation of any possible oscillations of the second seismic mass 55 (and, consequently, also of the first seismic mass 53), preventing them from overstepping a critical amplitude threshold that might cause damage to or failure of the sensor 50.

FIG. 3 shows a different embodiment of the sensor (here designated by the reference number 150). The sensor 150 comprises the semiconductor body 51 and a mobile structure 153 (hereinafter referred to as “seismic mass”), having an own mass M₃.

The seismic mass 153 is, for example, of semiconductor material (such as silicon or polysilicon) and extends parallel to the surface 51 a of the semiconductor body 51. The seismic mass 153 has a first lateral surface 153 a and a second lateral surface 153 b opposite to one another along the first axis X.

The seismic mass 153 is supported by the first spring assembly 57 (described previously with reference to FIG. 2 ) having a respective axis of deformation extending in a first direction of deformation 160′ parallel to the first axis X. In particular, the ends 57 a″, 57 b″ of the springs that form the spring assembly 57 are in contact with, and fixed with respect to, the first lateral surface 153 a and the second lateral surface 153 b, respectively.

Consequently, the seismic mass 153 is set, along the first axis X, between the first and second springs 57 a, 57 b of the spring assembly 57.

In addition, at least one second spring assembly 159 is present. With the sensor 150 in resting conditions, i.e., when the seismic mass 153 is not subject to an external force that causes a displacement thereof, the second spring assembly 159 is physically separate from the seismic mass 153; in a different operating condition of the sensor 150, when an external force acts on the seismic mass 153, causing a displacement thereof in the direction of the axis X, the seismic mass 153 comes into abutment against abutment regions of the second spring assembly 159.

The second spring assembly 159 is, for example, of semiconductor material (such as silicon or polysilicon) and has an elastic constant K₃ thereof different from the elastic constant K₁ of the first spring assembly 57 (for example, higher than the elastic constant K₁). The axis of deformation of the second spring assembly 159 extends parallel to the axis X and is staggered with respect to the axis of deformation of the first spring assembly 57. The second spring assembly 159 includes a first elastic element (spring) 159 a and a second elastic element (spring) 159 b.

In the embodiment described by way of example, both the first elastic element 159 a and the second elastic element 159 b are serpentine springs, i.e., strips arranged to form respective paths extending in a serpentine fashion (as described previously). Each elastic element 159 a, 159 b has a respective end 159 a′, 159 b′ coupled to the semiconductor body 51 and a respective end 159 a″, 159 b″ coupled to the seismic mass 153. In particular, each elastic element 159 a, 159 b of the second spring assembly 159 is coupled, via the respective end 159 a′, 159 b′, to a respective fixing element 164 coupled to the surface 51 a of the semiconductor body 51 (in particular, each fixing element 164 is fixed with respect to the surface of the semiconductor body 51).

In the embodiment of FIG. 3 , four elastic elements 159 a, 159 b are present, so that the equivalent elastic constant of the second spring assembly 159 is given by 4K₃. In general, for any number N3 of elastic elements of the second spring assembly 159, the equivalent elastic constant of the second spring assembly 159 is given by N3·K₃.

Each first elastic element 159 a has an extension, measured along the first axis X between the respective ends 159 a′ and 159 a″, equal to a first length L_(3a), and each second elastic element 159 b has an extension, measured along the first axis X between the respective ends 159 b′ and 159 b″, equal to a second length L₃b.

Each elastic element 159 a, 159 b of the second spring assembly 159 includes, in a position corresponding to the respective end 159 a″, 159 b″, a respective stopper element 166 a obtained by a terminal protrusion having a main extension parallel to the second axis Y (i.e., perpendicular to the direction of the oscillations of the seismic mass 153).

The seismic mass 153 has a recess, within which the stopper element 166 a extends. Portions of the seismic mass 153 that include said recess form a respective housing element 166 b for the stopper element 166 a. Each stopper element 166 a and the respective housing element 166 b form a respective abutment assembly 166.

Each stopper element 166 a has a first side wall 167 a and a second side wall 167 b, opposite to one another along the first axis X, while each housing element 166 b has a first side wall 167 c and a second side wall 167 d, opposite to one another along the first axis X and facing the first and second side walls 167 a, 167 b, respectively, of the respective stopper element 166 a. For each abutment assembly 166, the side walls 167 a, 167 c are at a distance equal to a first length L₃ from one another, whereas the side walls 167 b, 167 d are at a distance equal to a second length L₄ from one another.

Each stopper element 166 a operates in a similar way to the stopper element 66 a of FIG. 2 , whereas the housing element 166 b has a function similar to what has been described with reference to the housing element 66 b of FIG. 2 .

As better described hereinafter, in an operating condition, the seismic mass 153 and each elastic element 159 a, 159 b abut against one another via each stopper element 166 a and the respective housing element 166 b.

Furthermore, as has been described with reference to FIG. 2 , the first seismic mass 153 includes the first and second electrodes 68 a, 68 b, thus forming the measurement structure 68 already described with reference to FIG. 2 .

In addition, the blocking elements 70 are present, here facing the first and second lateral surfaces 153 a, 153 b of the seismic mass 153. In particular, at least one (in FIG. 3 , two) the first blocking element 70′ is at a distance L_(3block) from the first lateral surface 153 a, along the first axis X, and at least one (in FIG. 3 , two) second blocking element 70″ is at a distance L_(4block) from the second lateral surface 153 b, along the first axis X. It may be noted that the distance d_(c1) is greater than L₃+L_(3block), and the distance d_(c2) is greater than L₄+L_(4block).

In use, the sensor 150 is biased, as discussed previously, for carrying out measurement of the external force applied.

With the sensor 150 in the resting condition as shown in FIG. 3 , no external force is applied to the sensor 150, and therefore the seismic mass 153 is in the resting position. In particular, in the plane XY, the seismic mass 153 has a centroid B (in the resting condition, equal to the centroidal position B_(stat)). In addition:

-   -   the lengths L₃, L₄ are the same as one another and equal to the         stop length L_(stop);     -   the first and second distances d_(c1), d_(c2) are the same as         one another and equal to the distance at rest d_(rest);     -   the first length L_(1a) and the second length L_(1b) are the         same as one another and equal to the first length at rest         L_(1rest);     -   the first length L_(3a) and the second length L_(3b) are the         same as one another and equal to a second length at rest         L_(3rest); and the first distance L_(3block) and the second         distance L_(4block) are the same as one another and equal to the         maximum distance L_(blockmax).

FIG. 3A shows the sensor 150 in a first operating condition, where the external force (having the first force value F₁ less than the threshold force value F_(th)) is applied to the sensor 150. As discussed previously, there is a relative movement of the seismic mass 153 with respect to the semiconductor body 51. In particular, in the plane XY, the centroid B is displaced along the first axis X with respect to the centroid at rest B_(stat). In addition:

-   -   the first length L₁ is greater than the stop length L_(stop),         and the second length L₂ is less than the stop length L_(stop);     -   the first distance d_(c1) is less than the distance at rest         d_(rest), and the second distance d_(c2) is greater than the         distance at rest d_(rest);     -   the first length L_(1a) is less than the first length at rest         L_(1rest), and the second length L_(1b) is greater than the         first length at rest L_(1rest);     -   the first length L_(3a) and the second length L_(3b) are the         same as one another and equal to the length at rest L_(3rest);         and     -   the first distance L_(3block) is less than the maximum distance         L_(blockmax), and the second distance L_(4block) is greater than         the maximum distance L_(blockmax).

Therefore, considering by way of example N1=1, N3=1, in the first operating condition of FIG. 3A, the resonance pulsation ω_(res) of the sensor 150 is obtained according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{1}}{M_{3}}}$

FIG. 3B shows the sensor 150 in a second operating condition, where the external force applied to the sensor 150 has a second force value F₂ greater than, or equal to, the threshold value F_(th). For the same reasons as those described previously, the seismic mass 153 is displaced with respect to the resting position of FIG. 3 , and each stopper element 166 a bears upon the respective housing element 166 b (i.e., the seismic mass 153 is in contact with the stopper element 166 a of the second spring assembly 159). In particular, in the plane XY the centroid B has a displacement along the first axis X with respect to the position of the centroid at rest B_(stat) greater than in the case illustrated in FIG. 3A, and the seismic mass 153 is in direct physical contact with the second spring assembly 159. In addition:

-   -   the first length L₁ is twice the length L_(stop), and the second         length L₂ is zero;     -   the first distance d_(c1) is less than the distance at rest         d_(rest) (moreover, it is less than the first distance d_(c1) of         FIG. 3A), and the second distance d_(c2) is greater than the         distance at rest d_(rest) (moreover, it is greater than the         second distance d_(c2) of FIG. 3A);     -   the first length L_(1a) is less than the first length at rest         L_(1rest) (moreover, it is less than the first length L_(1a) of         FIG. 3A), and the second length L_(1b) is greater than the first         length at rest L_(1rest) (moreover, it is greater than the         second length L_(1b) of FIG. 3A);     -   the first length L_(3a) is less than the second length at rest         L_(3rest), and the second length L_(3b) is greater than the         second length at rest L_(3rest); and     -   the first distance L_(3block) is less than the maximum distance         L_(blockmax) (moreover, it is less than the first distance         L_(3block) of FIG. 3A), and the second distance L_(4block) is         greater than the maximum distance L_(blockmax) (moreover, it is         greater than the second distance L_(4block) of FIG. 3A).

Therefore, considering by way of example N1=1, N3=1, in the second operating condition of FIG. 3B, the resonance pulsation ω_(res) of the sensor 150 is obtained according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{1} + K_{3}}{M_{3}}}$

In particular, if the external force applied to the sensor 150 has a value greater than, or equal to, the maximum force value F_(max) (greater than the threshold force value F_(th)), the seismic mass 153 bears upon the first blocking element 70′ at a portion of the first lateral surface 153 a of the seismic mass 153. The blocking elements 70 therefore enable limitation of any possible oscillations of the seismic mass 153, preventing them from overstepping a critical threshold of amplitude that might cause damage to or failure of the sensor 150.

In addition, FIG. 3C shows a further embodiment of the sensor 150 (here designated by the reference 150′), similar to the one illustrated in FIG. 3 .

In particular, in FIG. 3C the seismic mass 153 encloses and delimits at least one through opening, or cavity, 180, where a second set 189 of springs extends (which takes the place of the second spring assembly 159 of FIG. 3 ). In particular, the seismic mass 153 has side walls 180 a, 180 b opposite to one another along the first axis X and directly facing the cavity 180.

The second set 189 of springs comprises a first spring (elastic element) 189 a and a second spring (elastic element) 189 b, each having an elastic constant KY thereof, for example higher than the elastic constant K₁ of the first spring assembly 57. With the sensor 150′ in the resting condition, i.e., when the seismic mass 153 is not subject to an external force that causes a displacement thereof, the second set 189 of springs is physically separate from the seismic mass 153. In a different operating condition of the sensor 150′, when an external force acts on the seismic mass 153, causing a displacement thereof in the direction of the axis X, the seismic mass 153 comes into abutment against the abutment regions 186 a of the second spring assembly 159.

Each spring 189 a, 189 b is a planar spring obtained with MEMS technology, in particular a spring including a strip (for example, of semiconductor material), extending in the plane XY and having a main extension parallel to the second axis Y and a width W₁ measured along the first axis X. The first spring 189 a develops between an end 189 a′ thereof and an end 189 a″ thereof, opposite to one another with respect to the second axis Y, and the second spring 189 b develops between an end 189 b′ thereof and an end 189 b″ thereof, opposite to one another with respect to the second axis Y. The ends 189 a′, 189 b′ are fixed with respect to respective fixing elements 184 coupled to the surface 51 a of the semiconductor body 51 (in particular, each fixing element 184 is fixed with respect to the surface of the semiconductor body 51 and extends in the cavity 180). The abutment regions 186 a are located at the ends 189 a″, 189 b″. In particular, the abutment regions 186 a are fixed with respect to the ends 189 a″, 189 b″ and have a width W₂, measured along the first axis X, greater than the width W₁. Alternatively, the abutment regions 186 a are portions of the springs 189 a, 189 b that have a width W₂, measured along the first axis X, greater than the width W₁.

With the sensor 150′ in the operating condition where an external force acts on the seismic mass 153 causing a displacement thereof in the direction of the axis X, one of the side walls 180 a, 180 b bears upon the abutment regions 186 a of the second spring assembly 159, causing a deflection (deformation) of the respective spring 189 a, 189 b along the first axis X. The lengths, measured along the axis X, between the abutment regions 186 a and the side walls 180 a, 180 b, are here identified by Ly, L₄, and are homologous to the lengths L₃, L₄ of FIG. 3 .

Each abutment region 186 a and the side walls 180 a, 180 b of the seismic mass 153 facing it therefore form a respective abutment assembly 186, which enables variation of the elastic constant of the sensor 150′, as has been described with reference to FIGS. 3-3B.

Furthermore, in the embodiment illustrated in FIG. 3C, the abutment regions 186 a have, in the plane XY, a round shape (i.e., a circular profile) in order to distribute better the mechanical stresses due to contact between the abutment regions 186 a and the side walls 180 a, 180 b of the seismic mass 153.

As in FIG. 3 , the blocking elements 70 are moreover present. The blocking elements 70 extend (see FIG. 3C) in the cavity 180. They are fixed with respect to the surface 51 a of the semiconductor body 51 (in particular, each blocking element 70 is fixed with respect to the respective fixing element 184) and face one of the side walls 180 a, 180 b of the seismic mass 153, from which they are separated by distances L_(3block)′, L_(4block)′, homologous to the distances L_(3block), L_(4block) of FIG. 3 .

In the embodiment of FIG. 3C, four springs 189 a, 189 b are present, so that the equivalent elastic constant of the second set 189 of springs is given by 4K₃. In general, for any number N3′ of springs of the second set 189 of springs, the equivalent elastic constant of the second set 189 of springs is given by N3′·K₃′.

FIG. 4 shows a different embodiment of the sensor (here designated by the reference number 250). The sensor 250 comprises the semiconductor body 51 and a mobile structure (hereinafter referred to as “seismic mass”) 253, having a mass M₄. The seismic mass 253 is, for example, of semiconductor material (such as silicon or polysilicon) and extends parallel to the surface 51 a of the semiconductor body 51. The seismic mass 253 has a through opening, or cavity, 262. The seismic mass 253 surrounds and delimits said cavity 262. The seismic mass 253 is externally delimited by a first lateral surface 253 a and a second lateral surface 253 b opposite to one another along the first axis X. The seismic mass 253 also has a third lateral surface 253 c and a fourth lateral surface 253 d, which are opposite to one another along the first axis X and directly face the cavity 262.

The seismic mass 253 is physically coupled to the semiconductor body 51 via at least one spring assembly 259, which extends in the cavity 262. The spring assembly 259 is, for example, of semiconductor material (such as silicon or polysilicon) and has a respective axis of deformation in a direction of deformation 260 parallel to the first axis X.

The spring assembly 259 includes a first spring (elastic element) 259 a and a second spring (elastic element) 259 b. In one embodiment (not illustrated), the springs 259 a, 259 b are planar springs obtained with MEMS technology, more in particular springs that include a plurality of turns that define a serpentine path (as discussed previously; in particular, the springs 259 a, 259 b include first and second portions similar to the ones defined previously). In a per se known manner, each turn is defined as the minimum ensemble of first and second portions of each spring 259 a, 259 b (having a turn length, not illustrated, measured along the first axis X), which, when replicated a number of times by translating it by the turn length in the direction of deformation 260, forms said spring 259 a, 259 b. In the embodiment of FIG. 4 , the springs 259 a, 259 b are planar springs obtained with MEMS technology, more in particular springs that include a plurality of turns. Each turn extends in the plane XY and includes a strip, for example, of semiconductor material, arranged to form a polygonal closed path (for example, a rectangular path comprising minor sides parallel to the first axis X and major sides parallel to the second axis Y).

The number of turns is equal to a total number of turns n_(6foldtot).

Both the first spring 259 a and the second spring 259 b of the spring assembly 259 have a respective end 259 a′ and a respective end 259 b″, opposite to one another along the first axis X. Each spring 259 a, 259 b is coupled, via the respective end 259 a′, 259 b′, to a respective fixing element 264 fixed with respect to the semiconductor body 51 (in particular, with respect to the surface 51 a of the semiconductor body 51). Each spring 259 a, 259 b of the second spring assembly 59 is moreover coupled, via the respective end 259 a″, 259 b″, to the seismic mass 253. In detail, the ends 259 a″, 259 b″ coupled to the seismic mass are in contact with the third lateral surface 253 c and the fourth lateral surface 253 d, respectively, of the seismic mass 253.

The first spring 259 a has an extension, measured along the first axis X between the end 259 a′ and the end 259 a″, equal to a first length L_(a). The second spring 259 b has an extension, measured along the first axis X between the end 259 b′ and the end 259 b″ equal to a first length L_(b). Moreover, each spring 259 a, 259 b includes at least one stopper element 266 a arranged so as to come into abutment against the seismic mass 253 in an operating condition of the sensor 250, as discussed more fully hereinafter. Said stopper element 266 a is a protrusion of each portion 259 a, 259 b, having a main extension parallel to the second axis Y, and extends within a recess of the seismic mass 253. In what follows the portion of the seismic mass 253 that includes said recess is referred to as housing element 266 b. The stopper element 266 a and the housing element 266 b form an abutment assembly 266.

Each stopper element 266 a has a first side wall 267 a and a second side wall 267 b, opposite to one another along the first axis X, while each housing element 266 b has a first side wall 267 c and a second side wall 267 d, which are opposite to one another along the first axis X and face the first side wall 267 a and the second side wall 267 b, respectively, of the respective stopper element 266 a. For each abutment assembly 266, the side walls 267 a, 267 c are at a distance equal to a first length L₅ from one another, while the side walls 267 b, 267 d are at a distance equal to a second length L₆ from one another.

For the spring 259 a of the spring assembly 259 there are identified a first region 261 a′, which includes the turns comprised between the end 259 a″ and the stopper element 266 a, and a second region 261 a″, which includes the turns comprised between the end 259 a′ and the stopper element 266 a. For the spring 259 b of the spring assembly 259 there are identified a first region 261 b′, which includes the turns comprised between the end 259 b″ and the stopper element 266 a, and a second region 261 b″, which includes the turns comprised between the end 259 b′ and the stopper element 266 a.

The length, measured along the axis X between the end 259 a″ and the stopper element 266 a is here identified by L_(a)′. It may be noted that, since in this example the stopper element 266 a has a rectangular shape, the length L_(a)′ is defined between the end 259 a″ and an axis passing through the centroid of the stopper element 266 a and lying parallel to the axis Y. The length, measured along the axis X between the end 259 a′ and the stopper element 266 a is here identified by L_(a)″. Likewise, the length L_(a)″ is defined between the end 259 a′ and the aforementioned axis passing through the centroid of the stopper element 266 a and lying parallel to the axis Y. The length, measured along the axis X between the end 259 b″ and the stopper element 266 a is here identified by L_(b)′. Likewise, the length L_(b)′ is defined between the end 259 b″ and the aforementioned axis passing through the centroid of the stopper element 266 a and lying parallel to the axis Y. The length, measured along the axis X, between the end 259 b′ and the stopper element 266 a is here identified by L_(b)″. Likewise, the length L_(b)″ is defined between the end 259 b′ and the aforementioned axis passing through the centroid of the stopper element 266 a and lying parallel to the axis Y.

For the spring 259 a, the sum of the length L_(a)′ and of the length L_(a)″ is therefore equal to the first length L_(a). For the spring 259 b, the sum of the length L_(b), and of the length L_(b)″ is therefore equal to the second length L_(b). In the embodiment described by way of example, for each portion 259 a, 259 b the lengths L_(a)′, L_(a)′, L_(b), and L_(b)′ are the same as one another in the resting condition of the sensor 250, and the number of turns present between the ends 259 a″, 259 b″ and the stopper element 266 a is equal to the number of turns present between the stopper element 266 a and the ends 259 a′, 259 b′. However, according to further embodiments, not illustrated, the lengths L_(a)′ and L_(a)″ may be, in the resting condition of the sensor 250, different from one another, and the number of turns present between the end 259 a″ and the stopper element 266 a may be different from the number of turns present between the stopper element 266 a and the end 259 a′. Likewise, the lengths L_(b), and L_(b)″ may be, in the resting condition of the sensor 250, different from one another, and the number of turns present between the end 259 b″ and the stopper element 266 a may be different from the number of turns present between the stopper element 266 a and the end 259 b′.

In addition, as has been described with reference to FIG. 2 , the seismic mass 253 includes the first and second electrodes 68 a, 68 b, which form the measurement structure 68.

Furthermore, the blocking elements 70 are present, facing the first and second lateral surfaces 253 a, 253 b of the seismic mass 253. In particular, at least one (in FIG. 4 , two) first blocking element 70′ is at a distance L_(5block), along the first axis X, from the first lateral surface 253 a of the seismic mass 253, and at least one (in FIG. 4 , two) second blocking element 70″ is at a distance L_(6block), along the first axis X, from the second lateral surface 253 b of the seismic mass 253.

In use, the sensor 250 is biased, as discussed previously, for carrying out measurement of the external applied force.

With the sensor 250 in the resting condition as shown in FIG. 4 , no external force is applied to the sensor 250, and therefore the seismic mass 253 is in the resting position.

In particular, in the plane XY, the seismic mass 253 has a centroid B that coincides, in the resting condition, with a position B_(stat). In addition:

-   -   the first and second lengths L₅, L₆ are the same as one another         and equal to the stop length L_(stop);     -   the first and second distances dci, d_(c2) are the same as one         another and equal to the distance at rest d_(rest);     -   the first length L_(a) and the second length L_(b) are the same         as one another and equal to a length at rest L_(rest); and     -   the first distance L_(5block) and the second distance L_(6block)         are the same as one another and equal to the maximum distance         L_(blockmax).

FIG. 4A shows the sensor 250 in a first operating condition, where the external force (having the first force value F₁ lower than the threshold force value F_(th)) is applied to the sensor 250. As discussed previously, there is a relative movement of the seismic mass 253 with respect to the semiconductor body 51, which causes a deformation of the spring assembly 259. In particular, in the plane XY, the centroid B is displaced along the first axis X with respect to the centroid at rest B_(stat). In addition:

-   -   the first length L₅ is greater than the stop length L_(stop),         and the second length L₆ is less than the stop length L_(stop);     -   the first distance d_(c1) is less than the distance at rest         d_(rest), and the second distance d_(c2) is greater than the         distance at rest d_(rest);     -   the first length L_(a) is greater than the length at rest         L_(rest), and the second length L_(b) is less than the length at         rest L_(rest); and     -   the first distance L_(5block) is less than the maximum distance         L_(blockmax), and the second distance L_(6block) is greater than         the maximum distance L_(blockmax).

In detail, since the stopper elements 266 a are not yet in direct physical contact with (i.e., they do not abut against) the respective housing elements 266 b, the stress is distributed over the entire spring assembly 259, and all the turns undergo deformation. In this operating condition, for each portion 259 a, 259 b the number of turns that undergo deformation is equal to n_(fold1), which is in turn equal to the total number of turns n_(foldtot) (n_(fold1)=n_(foldtot)).

As is in itself known, the elastic constant of a spring, having an axis of deformation, is dependent upon the number of turns of the spring and upon the length of the spring measured along the axis of deformation (in particular, it is inversely proportional both to the number of turns and to said length). Consequently, each spring 259 a, 259 b of the spring assembly 259 has, in the first operating condition, a first elastic constant K₄ depending upon the number n_(fold1) of turns effectively involved in deformation. In the embodiment of FIG. 4 , two springs 259 a, 259 b are present, so that the equivalent elastic constant of the spring assembly 259 in the first condition is given by 2K₄. In general, for any number N4 of springs of the spring assembly 259, the equivalent elastic constant of the spring assembly 259 in the first operating condition is given by N4·K₄.

Therefore, considering by way of example N4=1, in the first operating condition of FIG. 4A, the resonance pulsation ω_(res) of the sensor 250 is obtained by applying the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{4}}{M_{4}}}$

FIG. 4B shows the sensor 250 in a second operating condition, where the external force applied to the sensor 250 has a second force value F₂ greater than, or equal to, the threshold force value F_(th). For the same reasons as those described previously, the seismic mass 253 is displaced with respect to the resting position of FIG. 4 , and the stopper elements 266 a are in abutment against the respective housing elements 266 b. In particular, in the plane XY, the centroid B presents a displacement along the first axis X with respect to the centroid at rest B_(stat) greater than in the case illustrated in FIG. 4A, and the springs 259 a, 259 b of the spring assembly 259 are set in direct physical contact with the seismic mass 253. In addition:

-   -   the first length L₅ is twice the stop length L_(stop), and the         second length L₆ is zero;     -   the first distance d_(c1) is less than the distance at rest         d_(rest) (moreover, it is less than the first distance d_(c1) of         FIG. 4A), and the second distance d_(c2) is greater than the         distance at rest d_(rest) (moreover, it is greater than the         second distance d_(c2) of FIG. 4A);     -   the first length L_(a) is greater than the length at rest         L_(rest) (moreover, it is greater than the first length L_(a) of         FIG. 4A), and the second length L_(b) is less than the length at         rest L_(rest) (moreover, it is less than the second length L_(b)         of FIG. 4A); and     -   the first distance L_(5block) is less than the maximum distance         L_(blockmax) (moreover, it is less than the first distance         L_(5block) of FIG. 4A), and the second distance L_(6block) is         greater than the maximum distance L_(blockmax) (moreover, it is         greater than the second distance L_(6block) of FIG. 4A).

In detail, since the stopper elements 266 a are in direct physical contact with (i.e., in abutment against) the respective housing elements 266 b, the further stress (i.e., the difference between the stress applied and the minimum stress necessary for bringing the stopper elements 266 a into abutment against the respective housing elements 266 b) is distributed only over the turns of the second regions 261 a″, 261 b″ of the springs 259 a and 259 b, which undergo further deformation: in other words, these are the turns comprised between the stopper element 266 a and the end 259 a′ (respectively, 259 b′). In what follows, denoted by the reference n_(fold2) is the number of turns belonging to each second region 261 a″, 261 b″ of the springs 259 a and 259 b.

In the embodiment provided by way of example presented in FIG. 4 , we have n_(fold2)=n_(foldtot)/2. Consequently, each spring 259 a, 259 b of the spring assembly 259 has, in the second operating condition, a second elastic constant K₅ that depends upon the second number of turns n_(fold2) (the second elastic constant K₅ being higher than the first elastic constant K₄). In the embodiment of FIG. 4 , two springs 259 a, 259 b are present, so that the equivalent elastic constant of the spring assembly 259 in the second operating condition is given by 2K₅. In general, given any number N4 of springs of the spring assembly 259, the equivalent elastic constant of the spring assembly 259 in the second operating condition is given by N4·K₅.

Therefore, considering by way of example N4=1, in the second operating condition of FIG. 4B, the resonance pulsation ω_(res) of the sensor 250 is obtained according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{5}}{M_{4}}}$

In particular, if the external force applied to the sensor 250 has a value greater than, or equal to, the maximum force value F_(max) (greater than the threshold force value F_(th)), the seismic mass 253 bears upon the first blocking element 70′.

FIG. 5 shows a further embodiment of a micromechanical device 350 (referred to hereinafter as “sensor 350”) according to one aspect of the present disclosure. The sensor 350 comprises the semiconductor body 51, a first mobile structure (hereinafter “seismic mass”) 353 having a first mass M₅, and a second mobile structure (hereinafter “seismic mass”) 355 having a second mass M₆, for example greater than the first mass M₅. Both the first and second seismic masses 353, 355 are, for example, of semiconductor material (such as silicon or polysilicon) and extend parallel to the surface 51 a of the semiconductor body 51. The second seismic mass 355 encloses and delimits a first through opening, or cavity, 362. Moreover, the second seismic mass 355 includes: a first side wall 355 a and a second side wall 355 b, directly facing the first cavity 362 and opposite to one another along the first axis X; and a third side wall 355 c and a fourth side wall 355 d, opposite to one another along the first axis X, which delimit the second seismic mass 355 externally.

The first seismic mass 353 is completely contained within the cavity 362. The first seismic mass 353 has, in turn, a second through opening, or cavity 363. The first seismic mass 353 encloses and delimits the cavity 363. Moreover, the first seismic mass 353 includes: a first side wall 353 a and a second side wall 353 b, directly facing the cavity 363 and opposite to one another along the first axis X; and a third side wall 353 c and a fourth side wall 353 d, facing the first cavity 362 and opposite to one another along the first axis X. The first seismic mass 353 is physically coupled to the semiconductor body 51 via a first set of springs (similar to what has been described with reference to the first spring assembly 57 of FIG. 2 , and therefore designated in what follows as first spring assembly 57) completely contained in the second cavity 363. In particular, the fixing ends 57 a′, 57 b′ of both of the springs 57 a, 57 b are fixed with respect to a same fixing element 64 (which in turn extends in the second cavity 363, in a way fixed with respect to the semiconductor body 51, in particular to the surface 51 a of the semiconductor body 51). The ends 57 a″, 57 b″ of the springs 57 a, 57 b of the first spring assembly 57 are, instead, fixed with respect to the first side wall 353 a and the second side wall 353 b, respectively, of the first seismic mass 353.

The first and second seismic masses 353, 355 are physically coupled to one another via a second spring assembly 359 (e.g., to the spring assembly 59 of FIG. 2 ), which extends in the first cavity 362. In particular, the second spring assembly 359 comprises a first spring (elastic element) 359 a and a second spring (elastic element) 359 b. Each spring 359 a, 359 b is a planar spring obtained with MEMS technology, more in particular a spring having a plurality of turns that define a serpentine path.

The first spring 359 a develops between an end 359 a′ thereof and an end 359 a″ thereof, while the second spring 359 b develops between an end 359 b′ thereof and an end 359 b″ thereof. The ends 359 a′, 359 b′ are fixed with respect to the first side wall 355 a and second side wall 355 b, respectively, of the second seismic mass 355.

The spring 359 a has a length, measured along the axis X, between the ends 359 a′ and 359 a″, identified by the reference L_(4a); the spring 359 b has a length, measured along the axis X, between the ends 359 b′ and 359 b″, identified by the reference L₄b.

The ends 359 a″, 359 b″ are, instead, fixed with respect to the third side wall 353 c and fourth side wall 353 d, respectively, of the first seismic mass 353.

Each spring 359 a and 359 b has an elastic constant K₆ that is higher than the elastic constant K₁ of each portion 57 a, 57 b. In the embodiment of FIG. 5 , two springs 359 a, 359 b are present, so that the equivalent elastic constant of the second spring assembly 259 is equal to 2K₆. In general, given any number N5 of springs of the second spring assembly 359, the equivalent elastic constant of the second spring assembly 359 is given by N5·K₆.

Likewise, in the embodiment of FIG. 5 , two springs 57 a, 57 b are present, so that the equivalent elastic constant of the first spring assembly 57 is equal to 2K₁. In general, given any number N1 of springs of the first spring assembly 57, the equivalent elastic constant of the first spring assembly 57 is given by N1·K₁.

In addition, the first seismic mass 353 includes at least one third through opening, or cavity, 365. The first seismic mass 353 encloses and delimits the cavity 365. A measurement structure extends in the cavity 365. Said measurement structure is similar to the measurement structure 68 described with reference to FIG. 2 , and is therefore designated in what follows by the same reference number. In particular, the first seismic mass 353 includes at least one first electrode 68 a, which extends in the third cavity 365 and faces, and is set (along the first axis X) between, a first portion 68 b′ and a second portion 68 b″ of the measurement structure 68.

Furthermore, at least one first contact element 380 a and at least one second contact element 380 b are fixed with respect to the surface 51 a of the semiconductor body 51 and face the third side wall 355 c and the fourth side wall 355 d, respectively, of the second seismic mass 355. The first contact element 380 a is at a distance from the third side wall 355 c of the second seismic mass 355 equal to a first contact length L_(cont1), and the second contact element 380 b is at a distance from the fourth side wall 355 d of the second seismic mass 355 equal to a second contact length L_(cont2). The contact lengths L_(cont1), L_(cont2) are less than the value of distance at rest d_(rest) (d_(rest) is the distance at rest between the electrode 68 a and the electrode 68 b′, which is equal to the distance at rest between the electrode 68 a and the electrode 68 b″). Consequently, the second seismic mass 355 is set, along the first axis X, between the first and second contact elements 380 a, 380 b so that the first and second seismic masses 353, 355 can move with respect to the semiconductor body 51 in a direction of deformation 360 parallel to the first axis X.

Moreover, the blocking elements 70 are present, here facing the third and fourth side walls 353 c, 353 d of the first seismic mass 353. In particular, at least one (in FIG. 5 , two) first blocking element 70′ is at a first distance L_(7block) from the third side wall 353 c of the first seismic mass 353, along the first axis X, and at least one (in FIG. 5 , two) second blocking element 70″ is at a second distance L_(5block) from the fourth side wall 353 d of the first seismic mass 353, along the first axis X. The first capacitor distance d_(c1) is greater than L_(7block), and the second capacitor distance d_(c2) is greater than L_(8block).

In use, the sensor 350 is biased, as discussed previously, for carrying out measurement of the external force applied.

With the sensor 350 in the resting condition as shown in FIG. 5 , no external force is applied to the sensor 350, and therefore both the first and second seismic masses 353, 355 are stationary and in the resting position. In particular, the first seismic mass 353 has a first centroid B₁, and the second seismic mass 355 has a second centroid B₂. In the resting condition, in the plane XY, the first and second centroids B₁, B₂ coincide with one another and with a position at rest B_(stat). In addition:

-   -   the first and second contact lengths L_(cont1), L_(cont2) are         the same as one another and equal to a length at rest         L_(contrest);     -   the first and second distances d_(c1), d_(c2) are the same as         one another and equal to the distance at rest d_(rest);     -   the first length L_(1a) and the second length L_(1b) are the         same as one another and equal to the length at rest L_(1rest);     -   the first length L_(4a) and the second length L_(4b) are the         same as one another and equal to a length at rest L_(4rest); and     -   the first distance L_(7block) and the second distance L_(5block)         are the same as one another and equal to the maximum distance         L_(blockmax).

FIG. 5A shows the sensor 350 in a first operating condition, where the external force (having the first force value F₁ lower than a threshold force value F_(th)) is applied to the sensor 350. In a way similar to what has been discussed previously, there is a relative movement of the first and second seismic masses 253, 255 with respect to the semiconductor body 51. In particular, considering the greater stiffness of the second spring assembly 359 as compared to the stiffness of the first spring assembly 57, the first and second seismic masses 353, 355 move in a way fixed with respect to one another. In detail, both the first centroid B₁ and the second centroid B₂ are displaced, in the plane XY along the first axis X, with respect to the position of the centroid at rest B_(stat) and substantially coincide with one another. In addition:

-   -   the first contact length L_(cont1) is less than the length at         rest L_(contrest), and the second contact length L_(cont2) is         greater than the length at rest L_(contrest);     -   the first distance d_(c1) is less than the distance at rest         d_(rest), and the second distance d_(c2) is greater than the         distance at rest d_(rest);     -   the first length L_(1a) is greater than the first length at rest         L_(7rest), and the second length L_(1b) is less than the length         at rest L_(7rest);     -   the length L_(4a) and the length L_(4b) are substantially the         same as one another and substantially equal to the length at         rest L_(8rest); and     -   the first distance L_(7block) is less than the maximum distance         L_(blockmax), and the second distance L_(5block) is greater than         the maximum distance L_(blockmax).

Therefore, considering by way of example N1=1 and N5=1, in the first operating condition of FIG. 5A, the sensor 350 has a resonance pulsation ω_(res) according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{1}}{M_{5} + M_{6}}}$

FIG. 5B shows the sensor 350 in a second operating condition, where the external force applied to the sensor 350 has the second force value F₂ greater than, or equal to, the threshold force value F_(th). For the same reasons as those described previously, both the first and second seismic masses 353, 355 are displaced with respect to the resting position of FIG. 5 , and the second seismic mass 355 bears upon the first contact element 380 a at the third side wall 355 c of the second seismic mass 355. In particular, in the plane XY, both the first centroid B₁ and the second centroid B₂ present respective displacements along the first axis X with respect to the position at rest B_(stat) of the centroid greater than the displacements in the case illustrated in FIG. 5A, and the displacement of the first centroid B₁ with respect to the position at rest B_(stat) of the centroid is greater than that of the second centroid B₂ with respect to the position at rest B_(stat) of the centroid. Consequently, in the second operating condition, the second seismic mass 355 is fixed with respect to the semiconductor body 51, whereas the first seismic mass 353 can oscillate and move further. In addition:

-   -   the first contact length L_(cont1) is zero, and the second         contact length L_(cont2) is twice the length at rest         L_(contrest);     -   the first distance d_(c1) is less than the distance at rest         d_(rest) (moreover, it is less than the distance d_(c1) of FIG.         5A), and the second distance d_(c2) is greater than the distance         at rest d_(rest) (moreover, it is greater than the distance         d_(c2) of FIG. 5A);     -   the first length L_(1a) is greater than the first length at rest         L_(1rest) (moreover, it is greater than the length L_(1a) of         FIG. 5A), and the second length L_(1b) is less than the first         length at rest L_(1rest) (moreover, it is less than the length         L_(1b) of FIG. 5A);     -   the first length L_(4a) of the second spring assembly 359 is         less than the second length at rest L_(2rest), and the second         length L_(4b) is greater than the second length at rest         L_(2rest); and     -   the first distance L_(7block) is less than the maximum distance         L_(blockmax) (moreover, it is less than the first distance         L_(7block) of FIG. 5A), and the second distance L_(8block) is         greater than the maximum distance L_(blockmax) (moreover, it is         greater than the distance L_(8block) of FIG. 5A).

Therefore, considering by way of example N1=1 and N5=1, in the second operating condition of FIG. 5B, the resonance pulsation ω_(res) of the sensor 350 is obtained according to the following mathematical expression:

$\omega_{res} \approx \sqrt{\frac{K_{1} + K_{6}}{M_{5}}}$

The contact elements 380 therefore enable limitation of any possible oscillations of the second seismic mass 355 and cause deformation of the second spring assembly 359 thanks to the inertia of the first seismic mass 353 (which, in the second operating mode, is no longer fixed with respect to the second seismic mass 355), thus providing a threshold mechanism for modifying the elastic response of the sensor 350.

Furthermore, if the external force applied to the sensor 350 has a value greater than, or equal to, the maximum force value F_(max) (greater than the threshold force value F_(t)′), the first seismic mass 353 bears upon the first blocking element 70′ at a portion of the third side wall 353 c of the first seismic mass 353. In other words, the first distance L_(7block) is zero, and the second distance L_(8block) is twice the maximum distance L_(blockmax). The blocking elements 70 therefore enable limitation of any possible oscillations of the first seismic mass 353, preventing them from overstepping a critical threshold of amplitude that might cause damage to or failure of the sensor 350.

From an examination of the characteristics of the disclosure provided according to the present disclosure, the advantages that it affords are evident.

In particular, the present disclosure makes it possible to provide acceleration sensors that present a variable and/or nonlinear response to accelerations/decelerations.

This enables just one sensor to measure different ranges of accelerations/decelerations and therefore detect and discriminate events that are very different from one another. In detail, having a same sensor that measures both accelerations of a low value (for example, equal to 16 g or 32 g) and accelerations of a high value (for example, equal to 128 g) guarantees a saving in terms of power dissipated for its operation, of area of integration dedicated thereto, and of overall cost of the device that houses the sensor.

In greater detail, the sensor 50 has two seismic masses 53, 55 and two spring assemblies 57, 59. It is possible to measure low accelerations via the first seismic mass 53 that deforms the first spring assembly 57 (while the second seismic mass 55 is fixed with respect to the semiconductor body 51, and the second spring assembly 59 does not substantially undergo deformation). It is then possible to measure high accelerations when the seismic masses 53, 55 are in abutment against and are fixed with respect to one another, and contribute to causing deformation both of the first set 57 and of the second spring assembly 59. Moreover, the spring assemblies 57, 59 are not in direct physical contact with one another, and this improves the mechanical stability of the sensor 50 by reducing the stresses to which the spring assemblies 57, 59 are subjected in the event of shocks. In the sensor 50, the threshold mechanism that enables modification of the elastic response of the sensor 50 is given by the elements 66 a and 66 b. The physical contact that can, in use, occur between the elements 66 a and 66 b involves elements of a bulk type, which are able to withstand high stresses. Therefore, critical stresses are not reached, thus guaranteeing a better mechanical stability of the sensor 50.

The sensor 150 has, instead, one seismic mass 153 and two sets 57, 159 of springs. It is possible to measure low accelerations via the deformations of the first spring assembly 57 caused by the seismic mass 153 (while the second spring assembly 159 is not stressed), and it is possible to measure high accelerations via the deformations, caused by the seismic mass 153, of both of the sets 57, 159 of springs. The elastic response of the sensor 150 can moreover be easily calculated via FEM (Finite-Element Modelling) simulations, in a per se known manner. In particular, as illustrated in FIG. 6A, the signal generated by the sensor 150 presents, as the acceleration increases (positive values), a first rectilinear stretch having a first slope followed by a second rectilinear stretch having a second slope, lower than the first slope. The first and second stretches are joined together in a continuous way (i.e., there is no zero-degree discontinuity but only a first-degree discontinuity). The plot of the signal generated by the sensor 150 at negative accelerations (i.e., decelerations) is specular (in particular, symmetrical with respect to the origin) to that of positive accelerations.

The sensor 250 has a seismic mass 253 and a spring assembly 259. The nonlinear elastic response of the sensor 250 is obtained by the operation described of the stopper elements 266 a and housing elements 266 b, which modify, in use, the properties of the spring assembly 259: by reducing the number of turns of the spring assembly 259 that can withstand the stresses due to the external force applied (i.e., by reducing the number of active turns), the elasticity of the spring assembly 259 changes, and therefore the response of the sensor 250. In this case, as illustrated in FIG. 6B, the stiffness of the spring assembly 259 (and therefore, the overall stiffness of the sensor 250) presents, as the displacement of the seismic mass 253 increases, a first rectilinear stretch (which indicates a first stiffness) followed by a second rectilinear stretch (which indicates a second stiffness). These two stretches are separated from one another by a zero-degree discontinuity of the stiffness corresponding to the instant when the physical contact between the stopper elements 266 a and housing elements 266 occurs.

The sensor 350 has two seismic masses 353, 355 and two sets 57, 359 of springs. The threshold mechanism that enables measurement of different ranges of acceleration is given by the physical contact of the second seismic mass 355 with the contact elements 380 fixed with respect to the semiconductor body 51, which decouples the seismic masses 353, 355 and causes activation of the second spring assembly 359. The response at output from the sensor 350 as a function of the acceleration is similar to the one described with reference to FIG. 6A for the sensor 150.

Furthermore, the elastic elements of the sensors discussed previously have a main extension and direction of deformation parallel to the surface 51 a of the semiconductor body 51. In particular, the external force acts in the direction of deformation of the elastic elements. This enables distribution of the stresses on the elastic elements in an efficient way, thus reducing the likelihood of damage or failure thereof.

Finally, it is clear that modifications and variations may be made to the disclosure described and illustrated herein, without thereby departing from the scope of the present disclosure, as defined in the annexed claims.

In particular, the measurement structure 68 may be of an interdigitated type (i.e., it may include a plurality of first and second electrodes 68 a, 68 b facing one another to form an array), to improve measurement sensitivity. Moreover, the measurement structure 68 may be based upon an effect different from the capacitive one discussed previously. For instance, the measurement structure 68 may be a structure, of a type in itself known, that implements a detection of a resistive, piezoelectric, or optical type.

In addition, each stopper element 66 a (equivalently, 166 a and 266 a, and each contact element 380 a, 380 b) may include a crowned portion adapted to improve contact with, and reduce the risk of its adhesion to, the respective housing element 66 b (respectively, 166 b and 266 b, and the second mass 355) during mutual contact. In particular, the side walls of each stopper element may have a convex shape.

The abutment assemblies 66, 166, 266 may be in a number and occupy positions different from what has been described herein, as likewise the blocking elements 70.

The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

1. A device, comprising: a semiconductor body having a surface; a first mobile mass having a first wall opposite a second wall along a first direction, the first mobile mass having a third wall opposite a fourth wall along a second direction that is transverse to the first direction; a first elastic assembly extending between the first wall and the second wall of the first mobile mass, the first elastic assembly having a first end coupled to the semiconductor body; a first electrode extending in the second direction from the first mobile mass; a second electrode extending in the second direction from the first mobile mass; a first fixed electrode coupled to the semiconductor body; and a second fixed electrode coupled to the semiconductor body, the first and second fixed electrodes are spaced from each other and extend from the semiconductor body in a third direction transverse to the first direction and transverse to the second direction, the first electrode of the first mobile mass extending between the first and second fixed electrodes.
 2. The device of claim 1, wherein the first mobile mass includes a first internal cavity, the first and second walls of the first mobile mass being in the first internal cavity.
 3. The device of claim 2, wherein the first and second electrodes extending into the first internal cavity of the first mobile mass.
 4. The device of claim 2, wherein the first elastic assembly is in the first cavity and the first internal cavity includes a first and second recess in the third and fourth walls, respectively, portions of the first elastic assembly extends into the first and second recess.
 5. The device of claim 1, wherein the first mobile mass includes a first recess on the third wall and a second recess on the third wall, a second elastic assembly having portions extending into the first and second recess.
 6. The device of claim 1, wherein the first elastic assembly has a second end opposite the first end, the second end being coupled to the first mobile mass and being within the first mobile mass.
 7. The device of claim 1, wherein the first elastic assembly has a second end opposite the first end, the second end and the first end being within the first and second walls of the first mobile mass.
 8. The device of claim 7, wherein the first elastic assembly is within the first mobile mass, the first elastic assembly includes a first extension that extends away from the first electrode.
 9. The device of claim 1, wherein the first mobile mass includes a first cavity and a second cavity.
 10. The device of claim 9, comprising a second elastic assembly in the second cavity of the first mobile mass, the first elastic assembly being in the first cavity.
 11. A device, comprising: a first mobile mass having a plurality of first walls and a plurality of second walls, each first wall of the plurality of first walls being opposite each second wall of the plurality of second walls; a plurality of first elastic assemblies, each first elastic assembly of the plurality of first elastic assemblies extend between ones of the plurality of the first walls and ones of the plurality of the second walls of the first mobile mass, each of the plurality of first elastic assemblies has a first anchor; a plurality of first electrodes extending in a first direction from the first mobile mass; and a first fixed electrode and a second fixed electrode spaced from each other in a second direction that is transverse to the first direction, the first and second fixed electrodes extend in a third direction transverse to the first direction and transverse to the second direction, a first electrode of the plurality of first electrodes extend between the first and second fixed electrodes.
 12. The device of claim 11 wherein the first mobile mass includes a first internal cavity, the first and second electrodes extending into the first internal cavity.
 13. The device of claim 11 wherein the first mobile mass includes a first and second internal cavity, the first and second internal cavities are between the first and second electrodes.
 14. The device of claim 11, wherein the first mobile mass is between ones of the plurality of first elastic assemblies.
 15. The device of claim 11, comprising a first cavity in the first mobile mass, the plurality of first electrodes extending in the first cavity and towards each other.
 16. A micromechanical device, comprising: a semiconductor body having a surface; a first mobile structure having a first inner surface opposite a second inner surface; a first elastic element extending between the first and second inner surfaces of the first mobile structure, the first elastic assembly extending in a first direction coplanar to the surface of the semiconductor body, the first elastic element having a first end coupled with the semiconductor body; a second elastic element having a first end coupled with the first mobile structure; a first electrode extending in the first direction from an outer surface of the first mobile structure; a first fixed electrode coupled with the semiconductor body; and a second fixed electrode coupled with the semiconductor body, the first and second fixed electrodes spaced from each other and extending from the semiconductor body in a second direction transverse to the first direction, the first electrode of the first mobile structure extending between the first and second fixed electrodes.
 17. The micromechanical device of claim 16, wherein the second elastic element has a second end opposite the first end coupled with the semiconductor body.
 18. The micromechanical device of claim 16, comprising a second mobile structure coupled with the first mobile structure by the second elastic element. 